Activation of the renin-angiotensin system (RAS) and generation of angiotensin II (Ang II) have long been known to play a crucial role in fibrotic renal disease beyond the hemodynamic actions of this system. Ang II blockade has been one of the greatest therapeutic breakthroughs for a variety of renal and cardiovascular diseases in the past two decades. However, disease progression is slowed but not stopped. The therapeutic limitations of Ang II blockade leave other molecules unopposed to sustain disease progression. One of these culprits and the subject of this grant is renin, which is markedly elevated by Ang II blockade. The specific hypothesis is that renin, in addition to its role to convert angiotensinogen to Ang I, has novel receptor- mediated actions that could play a role in renal fibrosis. We base that hypothesis on the observations that 1) direct activation of the renin receptor in mesangial cells induces synthesis of the fibrogenic cytokine transforming growth factor (TGF)-B1 and profibrotic proteins, 2) Binding of renin to this receptor activates the mitogen-activated protein kinase-1 and -2 (ERK1 & ERK2) and subsequent signaling that leads to TGF-IJ induction. Importantly, this receptor-mediated pathway is independent of renin's role in Ang II generation. Moreover, the elevated renin induced by Ang II blockade is colocalized with the renin receptor in diseased glomeruli. The long-term goals are 1) to understand the possible role of receptor-mediated actions of renin in renal fibrosis and 2) to elucidate the molecular basis of these actions in disease. We propose to provide this new information by accomplishing the following Specific Aims: 1) To determine whether the receptor mediated profibrotic action of renin observed in vitro acts in vivo in experimental kidney disease models. 2) To identify the specific receptor-binding site on the surface of the renin structure and investigate the relationship between the receptor binding site and its enzymatic site in order to provide the molecular basis for the receptor-mediated actions of renin and understand the biological role of the dual effects of renin in fibrotic disease. Successfully carried out, our studies will provide an important proof-of concept that renin has novel receptor-mediated actions, in addition to its role in Ang II generation, which play a role in renal fibrosis. That renin has bifunctional actions via different binding sites will be of enormous importance to be considered for new treatments targeting RAS in renal fibrosis.